usb_mux: fix NULL checks in the usb_mux_chain
usb_mux: fix NULL checks in the usb_mux_chain

Some boards (servo_v4p1) don't assign mux devices to all type-c ports.
Ensure the mux device is valid before performing mux operations.

BUG=b:246828811
BRANCH=none
TEST=make buildall -j

Signed-off-by: Keith Short <f6d578f97033c25f1206cb57d52f358974c82954@chromium.org>
Change-Id: I009593d4ee94d412c6068588121b77af18402ba6
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3897790
Reviewed-by: Matthew Blecker <d18f4466f2802f9c78c5c14a42b8c21fdbfabfcd@chromium.org>
Code-Coverage: Zoss <345cf107e82c70959d46f59d98bddc98ce458aff@prod.google.com>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec